<!DOCTYPE html>
<html lang="zh-CN">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <title>优化之join | Manor Farm Mysql</title>
    <meta name="generator" content="VuePress 1.7.1">
    <link rel="icon" href="/manor-farm-mysql/pgmanor_mysql.svg">
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.11.1/dist/katex.min.css">
    <script language="javascript" type="text/javascript" src="https://cdn.bootcdn.net/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
    <script language="javascript" type="text/javascript" src="/manor-farm-mysql/js/MouseClickEffect.js"></script>
    <script language="javascript" type="text/javascript" src="/manor-farm-mysql/js/pgmanor-self.js"></script>
    <meta name="description" content="Mysql学习手册">
    <meta name="viewport" content="width=device-width,initial-scale=1,user-scalable=no">
    
    <link rel="preload" href="/manor-farm-mysql/assets/css/0.styles.7bfce8db.css" as="style"><link rel="preload" href="/manor-farm-mysql/assets/js/app.bbd54b43.js" as="script"><link rel="preload" href="/manor-farm-mysql/assets/js/3.33fa8ee0.js" as="script"><link rel="preload" href="/manor-farm-mysql/assets/js/1.e501e4e7.js" as="script"><link rel="preload" href="/manor-farm-mysql/assets/js/26.8b7e1943.js" as="script"><link rel="preload" href="/manor-farm-mysql/assets/js/8.73e907b5.js" as="script"><link rel="prefetch" href="/manor-farm-mysql/assets/js/10.31e485b8.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/11.bd87e4cf.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/12.0fe1282f.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/13.f1de335a.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/14.4a84a4ee.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/15.bc190a07.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/16.3f42a3ab.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/17.f1cad67c.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/18.e8063fee.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/19.068739ed.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/20.2b248747.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/21.4fd4cbc1.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/22.2683cc54.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/23.2e39bbe5.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/24.560a4164.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/25.26555323.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/27.0f4ac9a1.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/28.f89ec4c0.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/29.71c751c3.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/30.ad576991.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/31.2746a45e.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/32.b62e21d8.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/33.ae9d6c1c.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/34.53969cd0.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/35.3e1a8a01.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/36.eb74300d.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/37.16568451.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/38.9a3bc894.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/39.7aab5a75.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/4.c231685d.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/40.cd44603a.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/41.64d0c11a.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/42.3fb5d657.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/43.d4f19236.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/44.6abd5f31.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/45.eaf7b761.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/46.d891856e.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/5.0292b513.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/6.df2ba939.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/7.3b1acb14.js"><link rel="prefetch" href="/manor-farm-mysql/assets/js/9.5220a7da.js">
    <link rel="stylesheet" href="/manor-farm-mysql/assets/css/0.styles.7bfce8db.css">
  </head>
  <body>
    <div id="app" data-server-rendered="true"><div class="theme-container" data-v-2d5f533b><div data-v-2d5f533b><div id="loader-wrapper" class="loading-wrapper" data-v-d48f4d20 data-v-2d5f533b data-v-2d5f533b><div class="loader-main" data-v-d48f4d20><div data-v-d48f4d20></div><div data-v-d48f4d20></div><div data-v-d48f4d20></div><div data-v-d48f4d20></div></div> <!----> <!----></div> <div class="password-shadow password-wrapper-out" style="display:none;" data-v-59a94c5a data-v-2d5f533b data-v-2d5f533b><h3 class="title" style="display:none;" data-v-59a94c5a data-v-59a94c5a>Manor Farm Mysql</h3> <!----> <label id="box" class="inputBox" style="display:none;" data-v-59a94c5a data-v-59a94c5a><input type="password" value="" data-v-59a94c5a> <span data-v-59a94c5a>已加密!</span> <button data-v-59a94c5a>OK</button></label> <div class="footer" style="display:none;" data-v-59a94c5a data-v-59a94c5a><span data-v-59a94c5a><i class="iconfont reco-eye" data-v-59a94c5a></i> <a target="blank" href="https://pgmanor.gitee.io/blog/" data-v-59a94c5a>密钥</a></span> <span data-v-59a94c5a><i class="iconfont reco-theme" data-v-59a94c5a></i> <a target="blank" href="https://vuepress-theme-reco.recoluan.com" data-v-59a94c5a>vuePress-theme-reco</a></span> <span data-v-59a94c5a><i class="iconfont reco-copyright" data-v-59a94c5a></i> <a data-v-59a94c5a><span data-v-59a94c5a>pgmanor</span>
  
<!---->
2021
</a></span></div></div> <div class="hide" data-v-2d5f533b><header class="navbar" data-v-2d5f533b><div class="sidebar-button"><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" role="img" viewBox="0 0 448 512" class="icon"><path fill="currentColor" d="M436 124H12c-6.627 0-12-5.373-12-12V80c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12z"></path></svg></div> <a href="/manor-farm-mysql/" class="home-link router-link-active"><img src="/manor-farm-mysql/pgmanor_mysql.svg" alt="Manor Farm Mysql" class="logo"> <span class="site-name">Manor Farm Mysql</span></a> <div class="links"><div class="color-picker"><a class="color-button"><i class="iconfont reco-color"></i></a> <div class="color-picker-menu" style="display:none;"><div class="mode-options"><h4 class="title">Choose mode</h4> <ul class="color-mode-options"><li class="dark">dark</li><li class="auto active">auto</li><li class="light">light</li></ul></div></div></div> <div class="search-box"><input aria-label="Search" autocomplete="off" spellcheck="false" value=""> <!----></div> <nav class="nav-links can-hide"><div class="nav-item"><a href="/manor-farm-mysql/zh/guide/" class="nav-link router-link-active"><i class="iconfont reco-api"></i>
  指南
</a></div><div class="nav-item"><a href="/manor-farm-mysql/zh/fix/messageBoard.html" class="nav-link"><i class="iconfont reco-suggestion"></i>
  留言版
</a></div><div class="nav-item"><a href="https://pgmanor.gitee.io/blog/2021/01/10/secret-farm-mysql/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-eye"></i>
  文章密钥
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></div><div class="nav-item"><div class="dropdown-wrapper"><a class="dropdown-title"><span class="title"><i class="iconfont reco-qq"></i>
      联系
    </span> <span class="arrow right"></span></a> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>QQ在线</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="https://wpa.qq.com/msgrd?v=3&amp;uin=2307546059&amp;site=qq&amp;menu=yes" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  pgmanor
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></li><li class="dropdown-item"><h4>其他方式</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/manor-farm-mysql/zh/fix/contactUs.html" class="nav-link"><i class="iconfont undefined"></i>
  更多
</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><a class="dropdown-title"><span class="title"><i class="iconfont reco-other"></i>
      友链
    </span> <span class="arrow right"></span></a> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>博客</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="https://pgmanor.gitee.io/blog/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  pgmanor
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-subitem"><a href="https://blog.csdn.net/u011622109" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-csd"></i>
  csdn
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-subitem"><a href="https://www.iworkh.com" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  iworkh
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></li><li class="dropdown-item"><h4>Vue</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="https://cn.vuejs.org/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  Vue
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-subitem"><a href="https://v1.vuepress.vuejs.org/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  VuePress
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-subitem"><a href="https://vuepress-theme-reco.recoluan.com/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  vuepress-theme-reco
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></li></ul></div></div><div class="nav-item"><a href="/manor-farm-mysql/zh/fix/donate.html" class="nav-link"><i class="iconfont reco-Coding"></i>
  捐赠
</a></div><div class="nav-item"><a href="https://gitee.com/pgmanor/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-mayun"></i>
  Gitee
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></div> <!----></nav></div></header> <div class="sidebar-mask" data-v-2d5f533b></div> <aside class="sidebar" data-v-2d5f533b><div class="personal-info-wrapper" data-v-ca798c94 data-v-2d5f533b><img src="/manor-farm-mysql/pgmanor_mysql.svg" alt="author-avatar" class="personal-img" data-v-ca798c94> <h3 class="name" data-v-ca798c94>
    pgmanor
  </h3> <div class="num" data-v-ca798c94><div data-v-ca798c94><h3 data-v-ca798c94>30</h3> <h6 data-v-ca798c94>文章</h6></div> <div data-v-ca798c94><h3 data-v-ca798c94>26</h3> <h6 data-v-ca798c94>标签</h6></div></div> <hr data-v-ca798c94></div> <nav class="nav-links"><div class="nav-item"><a href="/manor-farm-mysql/zh/guide/" class="nav-link router-link-active"><i class="iconfont reco-api"></i>
  指南
</a></div><div class="nav-item"><a href="/manor-farm-mysql/zh/fix/messageBoard.html" class="nav-link"><i class="iconfont reco-suggestion"></i>
  留言版
</a></div><div class="nav-item"><a href="https://pgmanor.gitee.io/blog/2021/01/10/secret-farm-mysql/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-eye"></i>
  文章密钥
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></div><div class="nav-item"><div class="dropdown-wrapper"><a class="dropdown-title"><span class="title"><i class="iconfont reco-qq"></i>
      联系
    </span> <span class="arrow right"></span></a> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>QQ在线</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="https://wpa.qq.com/msgrd?v=3&amp;uin=2307546059&amp;site=qq&amp;menu=yes" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  pgmanor
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></li><li class="dropdown-item"><h4>其他方式</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="/manor-farm-mysql/zh/fix/contactUs.html" class="nav-link"><i class="iconfont undefined"></i>
  更多
</a></li></ul></li></ul></div></div><div class="nav-item"><div class="dropdown-wrapper"><a class="dropdown-title"><span class="title"><i class="iconfont reco-other"></i>
      友链
    </span> <span class="arrow right"></span></a> <ul class="nav-dropdown" style="display:none;"><li class="dropdown-item"><h4>博客</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="https://pgmanor.gitee.io/blog/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  pgmanor
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-subitem"><a href="https://blog.csdn.net/u011622109" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-csd"></i>
  csdn
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-subitem"><a href="https://www.iworkh.com" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  iworkh
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></li><li class="dropdown-item"><h4>Vue</h4> <ul class="dropdown-subitem-wrapper"><li class="dropdown-subitem"><a href="https://cn.vuejs.org/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  Vue
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-subitem"><a href="https://v1.vuepress.vuejs.org/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  VuePress
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li><li class="dropdown-subitem"><a href="https://vuepress-theme-reco.recoluan.com/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont undefined"></i>
  vuepress-theme-reco
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></li></ul></div></div><div class="nav-item"><a href="/manor-farm-mysql/zh/fix/donate.html" class="nav-link"><i class="iconfont reco-Coding"></i>
  捐赠
</a></div><div class="nav-item"><a href="https://gitee.com/pgmanor/" target="_blank" rel="noopener noreferrer" class="nav-link external"><i class="iconfont reco-mayun"></i>
  Gitee
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></div> <!----></nav> <ul class="sidebar-links"><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading open"><span>初入尘世</span> <span class="arrow down"></span></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/manor-farm-mysql/zh/guide/essentials/e-01-install.html" class="sidebar-link">安装</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-02-ddl-dcl.html" class="sidebar-link">DDL语句</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-03-dml.html" class="sidebar-link">DML语句</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-04-sort.html" class="sidebar-link">sort排序</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-05-group.html" class="sidebar-link">group分组</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-06-union.html" class="sidebar-link">union合并</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-07-join.html" class="sidebar-link">join</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-08-transaction.html" class="sidebar-link">事务</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-09-view.html" class="sidebar-link">视图</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-10-procedure.html" class="sidebar-link">存储过程</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-11-function.html" class="sidebar-link">函数</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-12-index.html" class="sidebar-link">索引</a></li><li><a href="/manor-farm-mysql/zh/guide/essentials/e-13-devops.html" class="sidebar-link">运维</a></li></ul></section></li><li><section class="sidebar-group depth-0"><p class="sidebar-heading"><span>千锤百炼</span> <!----></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/manor-farm-mysql/zh/guide/advanced/a-01-introduction.html" class="sidebar-link">本章介绍</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/a-02-prepare-data.html" class="sidebar-link">数据准备</a></li><li><section class="sidebar-group collapsable is-sub-group depth-1"><a href="/manor-farm-mysql/zh/guide/" class="sidebar-heading clickable router-link-active"><span>原理</span> <span class="arrow right"></span></a> <!----></section></li><li><section class="sidebar-group collapsable is-sub-group depth-1"><a href="/manor-farm-mysql/zh/guide/" class="sidebar-heading clickable router-link-active open"><span>优化</span> <span class="arrow down"></span></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-01-slow-query.html" class="sidebar-link">慢查询</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-02-mysqldumpslow.html" class="sidebar-link">mysqldumpslow</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-03-explain.html" class="sidebar-link">explain</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-04-explain-select-type.html" class="sidebar-link">explain之select_type</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-05-explain-type.html" class="sidebar-link">explain之type</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-06-explain-extra.html" class="sidebar-link">explain之extra</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-07-optimization.html" class="sidebar-link">优化思路和手段</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-08-optimization-join.html" aria-current="page" class="active sidebar-link">优化之join</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-09-optimization-orderby.html" class="sidebar-link">优化之排序</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-10-optimization-groupby.html" class="sidebar-link">优化之分组</a></li><li><a href="/manor-farm-mysql/zh/guide/advanced/optimization/a-11-optimization-struct.html" class="sidebar-link">结构优化</a></li></ul></section></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>百战成神</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <div class="password-shadow password-wrapper-in" style="display:none;" data-v-59a94c5a data-v-2d5f533b><h3 class="title" style="display:none;" data-v-59a94c5a data-v-59a94c5a>优化之join</h3> <!----> <label id="box" class="inputBox" style="display:none;" data-v-59a94c5a data-v-59a94c5a><input type="password" value="" data-v-59a94c5a> <span data-v-59a94c5a>已加密!</span> <button data-v-59a94c5a>OK</button></label> <div class="footer" style="display:none;" data-v-59a94c5a data-v-59a94c5a><span data-v-59a94c5a><i class="iconfont reco-eye" data-v-59a94c5a></i> <a target="blank" href="https://pgmanor.gitee.io/blog/" data-v-59a94c5a>密钥</a></span> <span data-v-59a94c5a><i class="iconfont reco-theme" data-v-59a94c5a></i> <a target="blank" href="https://vuepress-theme-reco.recoluan.com" data-v-59a94c5a>vuePress-theme-reco</a></span> <span data-v-59a94c5a><i class="iconfont reco-copyright" data-v-59a94c5a></i> <a data-v-59a94c5a><span data-v-59a94c5a>pgmanor</span>
  
<!---->
2021
</a></span></div></div> <div data-v-2d5f533b><main class="page"><div class="page-title" style="display:none;"><h1 class="title">优化之join</h1> <div data-v-3b7f5bdf><i class="iconfont reco-account" data-v-3b7f5bdf><span data-v-3b7f5bdf>pgmanor</span></i> <i class="iconfont reco-date" data-v-3b7f5bdf><span data-v-3b7f5bdf>2021-01-02</span></i> <i class="iconfont reco-eye" data-v-3b7f5bdf><span id="/manor-farm-mysql/zh/guide/advanced/optimization/a-08-optimization-join.html" data-flag-title="Your Article Title" class="leancloud-visitors" data-v-3b7f5bdf><a class="leancloud-visitors-count" style="font-size:.9rem;font-weight:normal;color:#999;"></a></span></i> <i class="iconfont reco-tag tags" data-v-3b7f5bdf><span class="tag-item" data-v-3b7f5bdf>join</span></i></div></div> <div class="theme-reco-content content__default" style="display:none;"><p>本文主要介绍下sql的关联join优化</p> <ul><li>为匹配的条件增加索引(减少内层表的循环匹配次数)</li> <li>永远用小结果集驱动大结果集(其本质就是减少外层循环的数据数量)</li> <li>无法使用索引，增大join buffer size的大小（一次缓存的数据越多，那么内层包的扫表次数就越少）。减少不必要的字段查询（字段越少，join buffer 所缓存的数据就越多）</li></ul> <div id="boxx" data-v-f4ca0dac><div data-v-f4ca0dac><p v-if="true" class="custom-block-title" data-v-f4ca0dac></p> <p v-if="true" data-v-f4ca0dac></p></div></div> <h2 id="_1-join操作">1.join操作</h2> <ul><li>left join，保留左表所有数据，左边没有数据设置为 null。</li> <li>right join，保留右表所有数据，游标没有数据设置为 null。</li> <li>inner join，取左右表数据的交集。</li></ul> <p>在多表关联时我们经常用到 join，那么它的底层原理是怎样的？是如何进行关联的？</p> <blockquote><p>不同的数据库不一样，有的数据库有哈希连接和或者合并连接。</p> <p>Oracle 和 MySQL 都使用了嵌套循环（Nested-Loop Join）的实现方式。</p></blockquote> <h2 id="_2-join算法">2.join算法</h2> <p>Nested-Loop Join 算法，需要区分驱动表和被驱动表，先访问驱动表，筛选出结果集，然后将这个结果集作为循环的基础，访问被驱动表过滤出需要的数据。</p> <blockquote><p>一个双层for 循环 ，通过循环外层表的行数据，逐个与内层表的所有行数据进行比较来获取结果</p></blockquote> <p><strong>查询sql</strong></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">select</span> teacher<span class="token punctuation">.</span>name<span class="token punctuation">,</span> school<span class="token punctuation">.</span>id 
<span class="token keyword">from</span> teacher 
<span class="token keyword">join</span> school <span class="token keyword">on</span>
school<span class="token punctuation">.</span>id <span class="token operator">=</span> teacher<span class="token punctuation">.</span>school_id 
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><strong>伪代码</strong></p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">for</span><span class="token punctuation">(</span><span class="token class-name">TeacherRow</span> u <span class="token operator">:</span> <span class="token class-name">TeacherRowList</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token class-name">SchoolRow</span> s ： <span class="token class-name">SchollRowList</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span>s<span class="token punctuation">.</span>user_id <span class="token operator">==</span> u<span class="token punctuation">.</span>id<span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h3 id="_2-1-simple-nested-loop-join">2-1.Simple Nested-Loop Join</h3> <p>Simple Nested-Loop Join，简单嵌套循环。这是最简单的方案，性能也一般。对内循环没优化。</p> <p><strong>示意图</strong></p> <p><img src="/manor-farm-mysql/images/advanced/optimization/optimization-join-simple.png" alt="Simple Nested-Loop Join"></p> <p><strong>特点</strong></p> <p>简单粗暴容易理解，就是通过双层循环比较数据来获得结果，但是这种算法显然太过于粗鲁，
如果每个表有1万条数据，那么对数据比较的次数=1万 * 1万 =1亿次，很显然这种查询效率会非常慢。</p> <p><strong>开销</strong></p> <p>假设驱动表(A表)N行，被驱动表(B表)M行</p> <ul><li>A表扫描1次</li> <li>B扫描了N次</li> <li>内循环共处理了N*M次</li> <li>总共读取记录数 N+N*M</li></ul> <h3 id="_2-2-index-nested-loop-join">2-2.Index Nested-Loop Join</h3> <p>Index Nested-Loop Join，索引嵌套循环。</p> <p><strong>示意图</strong></p> <p><img src="/manor-farm-mysql/images/advanced/optimization/optimization-join-index.png" alt="Index Nested-Loop Join"></p> <p><strong>特点</strong></p> <p>被驱动表有索引，不用整体扫描下被驱动表</p> <p><strong>开销</strong></p> <p>假设驱动表(A表)N行，被驱动表(B表)M行，索引B树的高度为 IndexHeight。</p> <ul><li>A表扫描1次</li> <li>B扫描了0次(有索引，直接命中)</li> <li>内循环共处理了N*IndexHeight次</li> <li>总共读取记录数 N+N*IndexHeight次</li></ul> <div class="custom-block tip"><p class="custom-block-title">注意</p> <ul><li>如果B的索引里包含了需要字段，那么不需要回表查询</li> <li>如果B中不含有需要的字段，还需要回表查询一步操作</li></ul></div> <h3 id="_2-3-block-nested-loop-join">2-3.Block Nested-Loop Join</h3> <p>Block Nested-Loop Join，块嵌套循环。是对Simple Nested-Loop Join索引的优化。</p> <blockquote><p>引入了join buffer区，可以批量处理，不用一条一条的循环。从而提高效率。</p></blockquote> <p><strong>示意图</strong></p> <p><img src="/manor-farm-mysql/images/advanced/optimization/optimization-join-block.png" alt="Index Nested-Loop Join"></p> <p><strong>特点</strong></p> <p>驱动表多条记录一起放到join buffer中，减少最外层循环次数</p> <p><strong>开销</strong></p> <p>假设驱动表(A表)N行，被驱动表(B表)M行，join buffer的大小join_buffer_size</p> <ul><li>A表扫描1次</li> <li>B表扫描了<code>(A表记录大小)/join_buffer_size</code></li> <li>内循环共处理了*<code>M*(A表记录大小)/join_buffer_size</code>次</li> <li>总共读取记录数 <code>(M+1)*(A表记录大小)/join_buffer_size</code>次</li></ul> <p>查看<code>Block Nested-Loop Join</code>是否开启，mysql8中默认开启的</p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">show</span> variables <span class="token operator">LIKE</span>  <span class="token string">'optimizer_switch%'</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>结果</p> <blockquote><p><code>block_nested_loop=on</code>表示开启</p></blockquote> <p><strong>查看join_buffer_size大小</strong></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">show</span> variables <span class="token operator">LIKE</span>  <span class="token string">'join_buffer_size%'</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><blockquote><p>默认值是值是<code>262144</code>，即<code>256k</code></p></blockquote> <h2 id="_3-示例">3.示例</h2> <h3 id="_3-1-建表">3-1.建表</h3> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">CREATE</span> <span class="token keyword">TABLE</span> <span class="token punctuation">`</span>teacher<span class="token punctuation">`</span> <span class="token punctuation">(</span>
  <span class="token punctuation">`</span>id<span class="token punctuation">`</span> <span class="token keyword">int</span> <span class="token operator">NOT</span> <span class="token boolean">NULL</span> <span class="token keyword">AUTO_INCREMENT</span> <span class="token keyword">COMMENT</span> <span class="token string">'id'</span><span class="token punctuation">,</span>
  <span class="token punctuation">`</span>name<span class="token punctuation">`</span> <span class="token keyword">varchar</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span> <span class="token keyword">COLLATE</span> utf8mb4_general_ci <span class="token keyword">DEFAULT</span> <span class="token boolean">NULL</span> <span class="token keyword">COMMENT</span> <span class="token string">'姓名'</span><span class="token punctuation">,</span>
  <span class="token punctuation">`</span>school_id<span class="token punctuation">`</span> <span class="token keyword">int</span> <span class="token keyword">DEFAULT</span> <span class="token boolean">NULL</span> <span class="token keyword">COMMENT</span> <span class="token string">'学校id'</span><span class="token punctuation">,</span>
  <span class="token keyword">PRIMARY</span> <span class="token keyword">KEY</span> <span class="token punctuation">(</span><span class="token punctuation">`</span>id<span class="token punctuation">`</span><span class="token punctuation">)</span>
<span class="token punctuation">)</span> <span class="token keyword">ENGINE</span><span class="token operator">=</span><span class="token keyword">InnoDB</span> <span class="token keyword">DEFAULT</span> <span class="token keyword">CHARSET</span><span class="token operator">=</span>utf8mb4 <span class="token keyword">COLLATE</span><span class="token operator">=</span>utf8mb4_general_ci<span class="token punctuation">;</span>


<span class="token keyword">CREATE</span> <span class="token keyword">TABLE</span> <span class="token punctuation">`</span>school<span class="token punctuation">`</span> <span class="token punctuation">(</span>
  <span class="token punctuation">`</span>id<span class="token punctuation">`</span> <span class="token keyword">int</span> <span class="token operator">NOT</span> <span class="token boolean">NULL</span> <span class="token keyword">AUTO_INCREMENT</span> <span class="token keyword">COMMENT</span> <span class="token string">'id'</span><span class="token punctuation">,</span>
  <span class="token punctuation">`</span>name<span class="token punctuation">`</span> <span class="token keyword">varchar</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span> <span class="token keyword">COLLATE</span> utf8mb4_general_ci <span class="token keyword">DEFAULT</span> <span class="token boolean">NULL</span> <span class="token keyword">COMMENT</span> <span class="token string">'学校名'</span><span class="token punctuation">,</span>
  <span class="token keyword">PRIMARY</span> <span class="token keyword">KEY</span> <span class="token punctuation">(</span><span class="token punctuation">`</span>id<span class="token punctuation">`</span><span class="token punctuation">)</span>
<span class="token punctuation">)</span> <span class="token keyword">ENGINE</span><span class="token operator">=</span><span class="token keyword">InnoDB</span> <span class="token keyword">DEFAULT</span> <span class="token keyword">CHARSET</span><span class="token operator">=</span>utf8mb4 <span class="token keyword">COLLATE</span><span class="token operator">=</span>utf8mb4_general_ci<span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div><blockquote><p>然后我们往<code>teacher</code>表生成1000条，<code>school</code>表生成100条数据。可以使用存储过程，也可以使用dbeaver的<code>Generate Mock Data</code>生成测试数据</p></blockquote> <h3 id="_3-2-没有索引">3-2.没有索引</h3> <p>我们先测试下，只有id上有索引的场景</p> <p><strong>left join</strong></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">explain</span>
<span class="token keyword">select</span> teacher<span class="token punctuation">.</span>name<span class="token punctuation">,</span> school<span class="token punctuation">.</span>id 
<span class="token keyword">from</span> teacher 
<span class="token keyword">left</span> <span class="token keyword">join</span> school
<span class="token keyword">on</span> teacher<span class="token punctuation">.</span>school_id <span class="token operator">=</span> school<span class="token punctuation">.</span>id
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><table><thead><tr><th>id</th> <th>select_type</th> <th>table</th> <th>partitions</th> <th>type</th> <th>possible_keys</th> <th>key</th> <th>key_len</th> <th>ref</th> <th>rows</th> <th>filtered</th> <th>Extra</th></tr></thead> <tbody><tr><td>1</td> <td>SIMPLE</td> <td>teacher</td> <td></td> <td>ALL</td> <td></td> <td></td> <td></td> <td></td> <td>1000</td> <td>100.0</td> <td></td></tr> <tr><td>1</td> <td>SIMPLE</td> <td>school</td> <td></td> <td>eq_ref</td> <td>PRIMARY</td> <td>PRIMARY</td> <td>4</td> <td>manor_farm_mysql.teacher.school_id</td> <td>1</td> <td>100.0</td> <td>Using index</td></tr></tbody></table> <blockquote><p><code>left join</code>，指定了驱动表(teacher)，被驱动表(teacher)</p></blockquote> <p><strong>right join</strong></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">explain</span>
<span class="token keyword">select</span> teacher<span class="token punctuation">.</span>name<span class="token punctuation">,</span> school<span class="token punctuation">.</span>id 
<span class="token keyword">from</span> teacher 
<span class="token keyword">right</span> <span class="token keyword">join</span> school
<span class="token keyword">on</span> teacher<span class="token punctuation">.</span>school_id <span class="token operator">=</span> school<span class="token punctuation">.</span>id
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><table><thead><tr><th>id</th> <th>select_type</th> <th>table</th> <th>partitions</th> <th>type</th> <th>possible_keys</th> <th>key</th> <th>key_len</th> <th>ref</th> <th>rows</th> <th>filtered</th> <th>Extra</th></tr></thead> <tbody><tr><td>1</td> <td>SIMPLE</td> <td>school</td> <td></td> <td>index</td> <td></td> <td>PRIMARY</td> <td>4</td> <td></td> <td>50</td> <td>100.0</td> <td>Using index</td></tr> <tr><td>1</td> <td>SIMPLE</td> <td>teacher</td> <td></td> <td>ALL</td> <td></td> <td></td> <td></td> <td></td> <td>1000</td> <td>100.0</td> <td>Using where; Using join buffer (hash join)</td></tr></tbody></table> <blockquote><p><code>right join</code>，指定了驱动表(school)，被驱动表(teacher)。被驱动表(teacher)<code>school_id</code>上没有索引，所以全表扫描。</p></blockquote> <p><strong>inner join</strong></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">explain</span>
<span class="token keyword">select</span> teacher<span class="token punctuation">.</span>name<span class="token punctuation">,</span> school<span class="token punctuation">.</span>id 
<span class="token keyword">from</span> teacher 
<span class="token keyword">join</span> school
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><table><thead><tr><th>id</th> <th>select_type</th> <th>table</th> <th>partitions</th> <th>type</th> <th>possible_keys</th> <th>key</th> <th>key_len</th> <th>ref</th> <th>rows</th> <th>filtered</th> <th>Extra</th></tr></thead> <tbody><tr><td>1</td> <td>SIMPLE</td> <td>school</td> <td></td> <td>index</td> <td></td> <td>PRIMARY</td> <td>4</td> <td></td> <td>50</td> <td>100.0</td> <td>Using index</td></tr> <tr><td>1</td> <td>SIMPLE</td> <td>teacher</td> <td></td> <td>ALL</td> <td></td> <td></td> <td></td> <td></td> <td>1000</td> <td>100.0</td> <td>Using join buffer (hash join)</td></tr></tbody></table> <blockquote><p><code>inner join</code>没有条件，会选择数据量小的表为驱动表(school)。被驱动表(teacher)没有指定关联关系，所以全表扫描。</p></blockquote> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">explain</span>
<span class="token keyword">select</span> teacher<span class="token punctuation">.</span>name<span class="token punctuation">,</span> school<span class="token punctuation">.</span>id 
<span class="token keyword">from</span> teacher 
<span class="token keyword">inner</span> <span class="token keyword">join</span> school
<span class="token keyword">on</span> teacher<span class="token punctuation">.</span>school_id <span class="token operator">=</span> school<span class="token punctuation">.</span>id
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><blockquote><p>多了on条件</p></blockquote> <table><thead><tr><th>id</th> <th>select_type</th> <th>table</th> <th>partitions</th> <th>type</th> <th>possible_keys</th> <th>key</th> <th>key_len</th> <th>ref</th> <th>rows</th> <th>filtered</th> <th>Extra</th></tr></thead> <tbody><tr><td>1</td> <td>SIMPLE</td> <td>teacher</td> <td></td> <td>ALL</td> <td></td> <td></td> <td></td> <td></td> <td>1000</td> <td>100.0</td> <td>Using where</td></tr> <tr><td>1</td> <td>SIMPLE</td> <td>school</td> <td></td> <td>eq_ref</td> <td>PRIMARY</td> <td>PRIMARY</td> <td>4</td> <td>manor_farm_mysql.teacher.school_id</td> <td>1</td> <td>100.0</td> <td>Using index</td></tr></tbody></table> <blockquote><p>多了on条件，<code>teacher.school_id = school.id</code>会发现两个表，其中teacher的<code>school_id</code>上没有索引。</p> <p>因为Nested-Loop Join外层循环要全表扫描，不需要索引，所以就选了<code>teacher</code>作为外层,作为驱动表了。</p></blockquote> <h3 id="_3-3-有索引">3-3.有索引</h3> <p>我们<code>teacher</code>的<code>school_id</code>加上索引后，我们再来观察下执行计划</p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">alter</span> <span class="token keyword">table</span> teacher <span class="token keyword">add</span> <span class="token keyword">index</span> index_school<span class="token punctuation">(</span>school_id<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>然后我们再观察下几个join的执行计划</p> <p><strong>left join</strong></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">explain</span>
<span class="token keyword">select</span> teacher<span class="token punctuation">.</span>name<span class="token punctuation">,</span> school<span class="token punctuation">.</span>id 
<span class="token keyword">from</span> teacher 
<span class="token keyword">left</span> <span class="token keyword">join</span> school
<span class="token keyword">on</span> teacher<span class="token punctuation">.</span>school_id <span class="token operator">=</span> school<span class="token punctuation">.</span>id
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><table><thead><tr><th>id</th> <th>select_type</th> <th>table</th> <th>partitions</th> <th>type</th> <th>possible_keys</th> <th>key</th> <th>key_len</th> <th>ref</th> <th>rows</th> <th>filtered</th> <th>Extra</th></tr></thead> <tbody><tr><td>1</td> <td>SIMPLE</td> <td>teacher</td> <td></td> <td>ALL</td> <td></td> <td></td> <td></td> <td></td> <td>1000</td> <td>100.0</td> <td></td></tr> <tr><td>1</td> <td>SIMPLE</td> <td>school</td> <td></td> <td>eq_ref</td> <td>PRIMARY</td> <td>PRIMARY</td> <td>4</td> <td>manor_farm_mysql.teacher.school_id</td> <td>1</td> <td>100.0</td> <td>Using index</td></tr></tbody></table> <blockquote><p><code>left join</code>，指定了驱动表(teacher)，被驱动表(teacher)</p></blockquote> <p><strong>right join</strong></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">explain</span>
<span class="token keyword">select</span> teacher<span class="token punctuation">.</span>name<span class="token punctuation">,</span> school<span class="token punctuation">.</span>id 
<span class="token keyword">from</span> teacher 
<span class="token keyword">right</span> <span class="token keyword">join</span> school
<span class="token keyword">on</span> teacher<span class="token punctuation">.</span>school_id <span class="token operator">=</span> school<span class="token punctuation">.</span>id
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><table><thead><tr><th>id</th> <th>select_type</th> <th>table</th> <th>partitions</th> <th>type</th> <th>possible_keys</th> <th>key</th> <th>key_len</th> <th>ref</th> <th>rows</th> <th>filtered</th> <th>Extra</th></tr></thead> <tbody><tr><td>1</td> <td>SIMPLE</td> <td>school</td> <td></td> <td>index</td> <td></td> <td>PRIMARY</td> <td>4</td> <td></td> <td>50</td> <td>100.0</td> <td>Using index</td></tr> <tr><td>1</td> <td>SIMPLE</td> <td>teacher</td> <td></td> <td>ref</td> <td>index_school</td> <td>index_school</td> <td>5</td> <td>manor_farm_mysql.school.id</td> <td>1</td> <td>100.0</td> <td></td></tr></tbody></table> <blockquote><p><code>right join</code>，指定了驱动表(school)，被驱动表(teacher)。被驱动表(teacher)<code>school_id</code>加了索引，所以key为<code>index_school</code>。</p></blockquote> <p><strong>inner join</strong></p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">explain</span>
<span class="token keyword">select</span> teacher<span class="token punctuation">.</span>name<span class="token punctuation">,</span> school<span class="token punctuation">.</span>id 
<span class="token keyword">from</span> teacher 
<span class="token keyword">join</span> school
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><table><thead><tr><th>id</th> <th>select_type</th> <th>table</th> <th>partitions</th> <th>type</th> <th>possible_keys</th> <th>key</th> <th>key_len</th> <th>ref</th> <th>rows</th> <th>filtered</th> <th>Extra</th></tr></thead> <tbody><tr><td>1</td> <td>SIMPLE</td> <td>school</td> <td></td> <td>index</td> <td></td> <td>PRIMARY</td> <td>4</td> <td></td> <td>50</td> <td>100.0</td> <td>Using index</td></tr> <tr><td>1</td> <td>SIMPLE</td> <td>teacher</td> <td></td> <td>ALL</td> <td></td> <td></td> <td></td> <td></td> <td>1000</td> <td>100.0</td> <td>Using join buffer (hash join)</td></tr></tbody></table> <blockquote><p><code>inner join</code>没有条件，会选择数据量小的表为驱动表(school)。被驱动表(teacher)没有指定关联关系，所以全表扫描。</p></blockquote> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">explain</span>
<span class="token keyword">select</span> teacher<span class="token punctuation">.</span>name<span class="token punctuation">,</span> school<span class="token punctuation">.</span>id 
<span class="token keyword">from</span> teacher 
<span class="token keyword">inner</span> <span class="token keyword">join</span> school
<span class="token keyword">on</span> teacher<span class="token punctuation">.</span>school_id <span class="token operator">=</span> school<span class="token punctuation">.</span>id
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><blockquote><p>多了on条件</p></blockquote> <table><thead><tr><th>id</th> <th>select_type</th> <th>table</th> <th>partitions</th> <th>type</th> <th>possible_keys</th> <th>key</th> <th>key_len</th> <th>ref</th> <th>rows</th> <th>filtered</th> <th>Extra</th></tr></thead> <tbody><tr><td>1</td> <td>SIMPLE</td> <td>school</td> <td></td> <td>index</td> <td></td> <td>PRIMARY</td> <td>4</td> <td></td> <td>50</td> <td>100.0</td> <td>Using index</td></tr> <tr><td>1</td> <td>SIMPLE</td> <td>teacher</td> <td></td> <td>ref</td> <td>index_school</td> <td>index_school</td> <td>5</td> <td>manor_farm_mysql.school.id</td> <td>1</td> <td>100.0</td> <td></td></tr></tbody></table> <blockquote><p>多了on条件，<code>teacher.school_id = school.id</code>会发现两个表，其中teacher的<code>school_id</code>上有索引，school的<code>id</code>也有所以。 所以就选了数据量小的表,作为驱动表了。</p></blockquote> <h2 id="_4-总结">4.总结</h2> <ul><li>join关联sql的驱动和被却动表选择，跟join的方式(left、right、inner)有关</li> <li>on条件字段是否有索引，会影响到驱动表的选择</li> <li>数据量的大小也影响驱动表的选择</li> <li>三种join算法(simple nested-loop join、index nested-loop join、block nested-loop join)</li> <li>当使用到了Using join buffer时，如何内存充足的场景下，可以将<code>join_buffer_size</code>设置大一点</li></ul></div> <footer class="page-edit" style="display:none;"><!----> <div class="last-updated"><span class="prefix">Last Updated: </span> <span class="time">2021-1-23 9:56:36 PM</span></div></footer> <!----> <!----> <!----></main> <!----></div></div></div></div><div class="global-ui"><div class="back-to-ceiling" style="right:1rem;bottom:6rem;width:2.5rem;height:2.5rem;border-radius:.25rem;line-height:2.5rem;display:none;" data-v-c6073ba8 data-v-c6073ba8><svg t="1574745035067" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg" p-id="5404" class="icon" data-v-c6073ba8><path d="M526.60727968 10.90185116a27.675 27.675 0 0 0-29.21455937 0c-131.36607665 82.28402758-218.69155461 228.01873535-218.69155402 394.07834331a462.20625001 462.20625001 0 0 0 5.36959153 69.94390903c1.00431239 6.55289093-0.34802892 13.13561351-3.76865779 18.80351572-32.63518765 54.11355614-51.75690182 118.55860487-51.7569018 187.94566865a371.06718723 371.06718723 0 0 0 11.50484808 91.98906777c6.53300375 25.50556257 41.68394495 28.14064038 52.69160883 4.22606766 17.37162448-37.73630017 42.14135425-72.50938081 72.80769204-103.21549295 2.18761121 3.04276886 4.15646224 6.24463696 6.40373557 9.22774369a1871.4375 1871.4375 0 0 0 140.04691725 5.34970492 1866.36093723 1866.36093723 0 0 0 140.04691723-5.34970492c2.24727335-2.98310674 4.21612437-6.18497483 6.3937923-9.2178004 30.66633723 30.70611158 55.4360664 65.4791928 72.80769147 103.21549355 11.00766384 23.91457269 46.15860503 21.27949489 52.69160879-4.22606768a371.15156223 371.15156223 0 0 0 11.514792-91.99901164c0-69.36717486-19.13165746-133.82216804-51.75690182-187.92578088-3.42062944-5.66790279-4.76302748-12.26056868-3.76865837-18.80351632a462.20625001 462.20625001 0 0 0 5.36959269-69.943909c-0.00994388-166.08943902-87.32547796-311.81420293-218.6915546-394.09823051zM605.93803103 357.87693858a93.93749974 93.93749974 0 1 1-187.89594924 6.1e-7 93.93749974 93.93749974 0 0 1 187.89594924-6.1e-7z" p-id="5405" data-v-c6073ba8></path><path d="M429.50777625 765.63860547C429.50777625 803.39355007 466.44236686 1000.39046097 512.00932183 1000.39046097c45.56695499 0 82.4922232-197.00623328 82.5015456-234.7518555 0-37.75494459-36.9345906-68.35043303-82.4922232-68.34111062-45.57627738-0.00932239-82.52019037 30.59548842-82.51086798 68.34111062z" p-id="5406" data-v-c6073ba8></path></svg></div><!----><!----><APlayer audio="" fixed="true" mini="true" theme="#f9bcdd" loop="loop" order="random" preload="auto" volume="0.1" mutex="true" lrc-type="0" list-folded="true" list-max-height="250" storage-name="vuepress-plugin-meting" id="aplayer-fixed"></APlayer></div></div>
    <script src="/manor-farm-mysql/assets/js/app.bbd54b43.js" defer></script><script src="/manor-farm-mysql/assets/js/3.33fa8ee0.js" defer></script><script src="/manor-farm-mysql/assets/js/1.e501e4e7.js" defer></script><script src="/manor-farm-mysql/assets/js/26.8b7e1943.js" defer></script><script src="/manor-farm-mysql/assets/js/8.73e907b5.js" defer></script>
  </body>
</html>
